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Green UPS Tier IV data center water-side economizers

The facilities team at United Parcel Service's data center in Alpharetta, Ga., have green data center operations down to a science. Learn how they did it in this two-part case study.

*Editor's note: This is a two-part case study. Part two is about UPS's water-side economizer use and other power-saving cooling strategies. Part one focuses on how UPS identified wasted cooling, and in doing so cut out the CRAHs.

Free data center cooling in Georgia
The Windward data center has two 1,000-ton centrifugal chillers and two 800-ton absorption chillers. The data center also has a 650,000-gallon thermal storage tank with redundant water sources (a well and city water for backup). The thermal storage tank was designed to provide about 20 hours of emergency cooling, but Parrino's team also uses it for energy cost management.

In 2000, United Parcel Service installed a plate-and-frame heat exchanger to take advantage of outside air temperatures to cool its chilled water. Also known as water-side economizing, the practice saves energy by allowing data centers to turn off chillers, and green data center experts have given it a lot of attention lately.

Unfortunately, most people don't take advantage of free cooling because they aren't in a region that stays cold long enough for the system to pay for itself or lack the automation to manage going on and off the plate-and-frame heat exchanger. But UPS has solved both of those problems.

For starters, Parrino's staff raised the temperature of the chilled-water loops from the designed temperature of 45 degrees Fahrenheit. It now modulates between 52 degrees and 58 degrees Fahrenheit. The lower temperatures are needed during high-humidity days (i.e., 100% humidity when it rains) to maintain the interior relative humidity between the nominal 40% to 55%. During the winter months when the outside air is drier, Windward can use higher temperatures.

Further, getting on and off the plate-and-frame heat exchanger is easier with a thermal storage tank. As the chiller shuts down and a condenser water loop is lowered, the thermal storage tank provides uninterruptible cooling to the data processing equipment.

Windward is somewhat of a mixed-use facility – about 125 people (support personnel) work at the data center. The increase in chilled water temperatures has not affected human comfort in any way. (For more on the difference between latent and sensible cooling, see the "Servers don't sweat: Raise your chilled-water temp" podcast.)

Higher chilled-water temperatures enable United Parcel Service to extend its use of free cooling dramatically. In 2007 the data center got used the plate-and-frame heat exchanger for the last time on May 18 and switched free cooling back on for the first time on Oct. 11. During this seasonal transition period, nighttime temperatures get low enough but the days are warm. To extend time on the plate-and-frame heat exchanger even further during the seasonal transition periods, the thermal storage tank provides ride-through during the warm afternoons and is then re-charged during cool evenings. The entire process is automated; no human intervention is required.

As of late November 2007, Windward is on plate about 90% of the time and will remain that way through the better part of April. That's five months of free cooling—in Atlanta, no less. Northern latitudes should enjoy even longer free cooling periods.

Based on operating 73 days per year, the plate-and-frame heat exchanger project was projected to pay for itself in two and a half years, saving 4 cents per kilowatt-hour. It saved $88,000 annually. The winter of 2000 was especially cold, and in the first year of operation, the project paid for itself .

According to Parrino, switching on and off a plate-and-frame heat exchanger would be a messy job without thermal storage tank and solid automation software. He says the Windward building automation system from Kennesaw, Ga.-based Automated Logic Corp. is a story in itself.

In 1995 the system was installed with the building, and the plan was to bring all building systems under a single interface. "It was a pretty advanced system in 1995, even more so today," Parrino said. "Manufacturers want to give you a PC for your UPS system, one for the generator or a chiller. Our system interfaces with all of these third-party devices."

The system provides chiller, pumps, and cooling-tower rotations and manages the thermal storage tank. It also gives Windward visibility into outside air temperature and humidity conditions to determine when a data center can use outside cooling.

How peak-load shedding gets done
Automated Logic's system also helps United Parcel Service shed its power load during high-demand peaks in the summer. United Parcel Service is on a real-time pricing plan with its utility Georgia Power. The price can range from 4 cents per kilowatt-hour in the morning and jump to 8 cents in the afternoon when there are moderate summertime temperatures. Costs for afternoon peak-load times in the month of August exceeded 30 cents per kilowatt-hour on days when the outside temperature surpassed 100 degrees Fahrenheit.

In order to minimize costs, Windward switches to "plant economy" mode during the summer peak-load periods. Plant-economy mode effectively shuts down the 630 kW chiller plant (including chillers, cooling tower fans, primary pumps, tower pumps) and cools the data center using the stored 45 degrees Fahrenheit thermal energy in its 650,000-gallon thermal storage tank. The facility then runs chillers at night when the cost per kilowatt-hour is around 4 cents—recharging the tank during off-peak hours.

Running the chillers at night is an effective energy reduction strategy as well, since outside wet-bulb temperatures are typically lower than they are during daytime hours. A lower wet-bulb temperature allows more efficient removal of heat via cooling towers. This reduces the condenser water temperature, also reducing "lift" in the chiller and enabling it to run more efficiently.

The thermal storage tank only used to provide six to eight hours of cooling capacity on a 95 degree day, with 40% thermal capacity remaining. Raising the chilled-water set point has increased the ton-hours capacity to approximately 16 hours of cooling with 50% thermal capacity remaining. The tank is never completely discharged so it can continue to provide thermal backup in case of a chiller problem. Finally, it removes stress on Georgia Power's peak power capacity infrastructure.

Eco-awareness at United Parcel Service
It's not clear that companies are in a green mood. In "IT priorities in 2008: A truly new year," reported on a broad survey of TechTarget Inc. members. Results indicated that in 2008, green computing remains a minor initiative. For the moment, many companies have simply fed the energy-consumption beast by building new data centers to provide additional raw power for an increasing number of servers.

United Parcel Service is a notable exception. According to Parrino, the company's founder, James Casey, embedded the principle to always be "constructively dissatisfied" and constantly to seek opportunities to improve efficiency. United Parcel Service has actively gone green in its data center and views its efforts with a broader impact on the environment.

"When you look at electrical costs of $100,000 a month in our $47 billion company, it's not a lot of money for one building," Parrino said. "But going beyond that as a good corporate citizen, UPS has learned to manage the consumption of energy in all aspects of their business. The data center is no exception. Energy sources in the Southeast are plentiful, but not necessarily renewable. As a renewable generating source in Georgia, wind is not good, solar is marginal, and geothermal and hydro simply aren't available. Typical electrical generating sources are 50% coal, and 50% natural gas."

United Parcel Service also equates energy efficiency with increasing the useful service life of the data center. "All these data centers around the country are running out of power and cooing and are having to expand," Parrino said. "Becoming energy efficient is a great payback when you don't have to expand into additional infrastructure."

ABOUT THE AUTHOR: Matt Stansberry is's senior site editor. Write to him about your data center concerns at

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